logging off automatically

When I leave the back end of my WordPress sites I get automatically and immediately logged off. This does not appear to be the case with DirectoryPress (or this Forum for that matter) I have to go to the log off button and click on it in order to log off. Is there anyway I could configure something that would get me automatically logged off when I leave without clicking the logoff button. If everyone else gets immediately logged off what do I need to do to fix it. thanks